Multiple NICs in the same subnetwork

This document explains how to configure different network layouts that involve having multiple network interfaces in the same subnet. Each layout has specific setup steps.

In the examples we use:

• Host 1 to represent the storage server
• while Host 2 represents the machine that communicates with the storage server.

IMPORTANT NOTE

As a rule, we suggest that you avoid using network layouts that need multiple interfaces in the same subnet, because they may cause some network services to be unstable. Additionally, it might result in routing issues – for example, a packet sent through eth0 may get a reply from eth1. If you want to improve network performance, please consider using bonding or iSCSI multipathing scenarios instead.

Scenario 1

Host 1 and Host 2 are connected through a switch. They have multiple network interfaces in the same subnet to communicate with each other.

Multiple NICs in the same subnetwork Scenario1.png

CONFIGURATION

• The need for static routing depends on the service being used.
• If Host 2 initiates the connection, Host 1 may not need static routing, but it is better to set it up for more reliability.
• If Host 1 initiates the connection, Host 1 needs static routing. This also applies if Host 2 is another storage server.

POSSIBLE ISSUES

Without static routing, Host 1 will use only the interface that comes first in the routing table (for example, eth1) for all connections it starts.

The interface that comes first in the routing table is essential for Host 1 to initiate any connections. If it fails, Host 1 will lose this ability.

You can prevent this problem by configuring static routing for each interface.

To configure static routing in the GUI, go to "System Settings", then select the "Network" tab and click the "Add static routing" button in the "Static routing manager" section.

The 'New route' window will appear. The fields should be filled in with the appropriate values.

Network/Host IP: enter the IP address of the network card of the target host
Netmask: when we use a static route for a specific end host, the subnet mask is 32-bit (255.255.255.255)
Interface: from the drop-down menu, select the interface from which communication is to be established.

Below is an example static route configuration for three NICs in the same subnet.

Scenario 2

Host 1 and Host 2 are connected through a switch. They have multiple network interfaces in the same subnet to communicate with each other. Additionally,  Host 1 also has an interface for WAN access.

CONFIGURATION

• We strongly suggest that you use a different subnet for the WAN interface than the other interfaces. In such a case, this layout will be similar to Layout 1 and you can follow the same setup steps.
• But, unlike Layout 1, this layout always needs a gateway on the WAN interface (see the image below)
• Please check the “Possible issues” section for Layout 1 for problems that may also happen in this layout.

To set up the gateway, please press CTRL+ALT+N on the system console and choose the interface. Then, select "Gateway."

Scenario 3

Host 1 and Host 2 are directly connected. For better connectivity with this host, there are several network interfaces within the same subnetwork.

CONFIGURATION

• This scenario requires static routing to be set up on Host 1 and, depending on the operating system, on Host 2 as well.
• A local output interface must be assigned to every remote IP. This rule applies to both hosts.

POTENTIAL ISSUES

If Host 1 does not use static routing, it will always initiate connections through the interface that appears first in the routing table (in the example below, eth1).

You can prevent this issue by configuring static routing for each interface.